Hydrodynamic cavitation is a physical phenomenon that involves the formation and collapse of cavities within a liquid. The cavities (bubbles) are often filled with a vapor-gas mixture and are typically created by reducing a fluid's local pressure. The vapor bubbles, once formed, cannot hold their shape, being formed by fluid walls, and their rapid collapse creates mixing, emulsification, dispersion, and kinetic effects which may be useful. Since the fluid is typically near its particular phase transformation at the inception of cavitation, the process can be used for particle dispersion, atomization, and emulsion formation. Sufficient agitation can be imparted such that full vaporization can be accomplished, if so desired. In addition, the cavitation of certain substances create desired chemical reactions. For example, the cavitation of water produces OH radicals and peroxides which serve to attack any biological contaminants within the water, providing a level of disinfection of the water through the occurrence of cavitation.
The mechanism of the bubble collapse, as described in the field, is that a “finger” of fluid is pulled through the surface of the bubble, which has lower pressure than the surrounding fluid, from a single point (or a small local region) of the bubble's surface. This finger forms a micro-jet of fluid, which strikes the opposing surface of the bubble with great force and speed (measured as being supersonic). This collision creates forces that can be disruptive to the chemical structure of the fluid. Sonochemistry involves the studying of such forces to facilitate various chemical reactions and efforts, including atomization.
Traditionally, hydrodynamic cavitation is created by pumping a fluid through a venturi of some sort. The restrictive portion of the venturi creates the necessary local velocity increase and corresponding pressure drop, which precipitates cavitation. A venturi may be formed by a fixed diameter throat that is smaller in diameter than an upstream inlet, smoothly transitioned from the inlet and onward to the outlet, or it may be formed by a simple restriction placed inside a fluid flow field which has a similar effect of raising the fluid's velocity and thus lowering its pressure. This occurs in accordance with Bernoulli's principle which states that for steady flow of an incompressible fluid the pressure will decrease if the velocity increases, and visa versa. In the case of a venturi, the restriction causes an increase in the fluid's local velocity and thus lowers its pressure. This can be accomplished through the use of a simple restrictive element as well, although flow losses will be greater. Conventional venturi systems are held fixed and a fluid is passed through them.
The pressure reduction created by venturis (or restrictive elements in passages) has been utilized to create cavitation in fluids by reducing a fluid's local pressure to a level below its vaporization pressure, causing vapor bubbles to form in the fluid. These bubbles collapse, in the case of a venturi, once the pressure of the fluid recovers downstream of the venturi throat, the higher relative fluid pressure causing the vapor bubble to implode upon itself with great force.
It is also observed in experiments that do no involve centrifugal or centripetal forces such as experiments involving stretching a fluid using a piston and cylinder or by cooling a fluid in a fixed volume that vapor bubbles may be created within a fluid through the application of tensile stresses to the fluid. Due to the weak molecular bonds existing within a fluid, once the tensile strength of the fluid is exceeded, the fluid “fractures” and forms a vapor bubble. If the tensile stress within the fluid is removed in a sufficiently rapid fashion, the bubble will collapse, completing the cavitation cycle.